1. Field of the Invention
The present invention relates to a packaging container.
2. Description of the Related Art
Conventionally, packaging containers used for holding liquid foods such as soft drinks and milk have been manufactured in a manner in which a web-like packaging material is formed into a brick-like shape, for example.
FIG. 1 is a sectional view of a conventional packaging material.
As shown in this drawing, a packaging material 11 is composed of a paper substrate 13, a layer 14 which is the innermost layer when the packaging material 11 is shaped into a package container (hereinafter simply referred to as the "innermost layer"), a layer 15 which is the outermost layer when the packaging material 11 is shaped into a package container (hereinafter simply referred to as the "outermost layer"), a gas barrier layer 16 formed between the paper substrate 13 and the innermost layer 14, and an adhesive layer 27. The paper substrate 13 is made of paperboard or a similar material, and the gas barrier layer 16 is bonded thereto by the adhesive layer 27.
The opposite side edges of the packaging material 11 are overlapped with each other and then joined together by heat fusion in the longitudinal direction to obtain a tubular packaging material. The tubular packaging material is then continuously transferred downward while a liquid food is supplied into it from its top. Then, the tubular material is pressed from both sides thereof and is sealed laterally at predetermined intervals so as to obtain pillow shaped containers, which are then formed into a brick-like shape.
A strip tape 25 is adhered to the sealed portion longitudinally extending along the inner surface of each packaging container (hereinafter referred to as "longitudinal sealing portion") to prevent air from entering the packaging container from a side edge of the packaging material 11 and to prevent liquid food from permeating the paper substrate 13.
Packaging containers, the packaging material 11 of which has the innermost layer 14 made of a polyolefin resin, do not have sufficient ability to maintain the flavors of foods. This is because liquid foods contained in the containers contact the polyolefin resin, which adsorbs flavoring ingredients of the liquid foods over time.
In order to increase the flavor-retaining ability of each container, the polyolefin resin layer may be coated with a non-olefin flavor-retaining resin which serves as the innermost layer 14. In this case, the strip tape 25 is made of a non-polyolefin resin which can be easily joined to the above-described flavoring-retaining resin by heat fusion. The strip tape 25 is adhered to the longitudinal sealing portion using heat and pressure.
When the innermost layer 14 of the web-like package material is made of a flavor-retaining resin, the resin is joined to a polyolefin resin forming the outermost layer 15 when the overlapped side edges of the packaging material 11 are fused and joined in the longitudinal direction. However, since the flavor-retaining resin and the polyolefin resin are difficult to join with each other, only a poor seal can be obtained.
This problem can be solved by joining the flavor-retaining resin and the polyolefin resin by fusion after activating the sealing portion of the surface of the flavor-retaining resin by a corona discharge treatment, an ozone treatment, or a flame treatment, or by joining the flavor-retaining resin and the polyolefin resin by fusion at a high temperature.
However, a sufficient seal cannot be obtained even when the flavor-retaining resin is subjected to corona discharge treatment, ozone treatment or flame treatment.
When the flavor-retaining resin and the polyolefin resin are fused at a high temperature to join them together, the manufacturing costs increase due to the necessity of an apparatus for high-temperature fusion.
It is possible to join the flavor-retaining resin forming the innermost layer 14 and the polyolefin resin forming the outermost layer 15 with an adhesive made of an ethylene - vinyl acetate copolymer (EVA), polyethylene chloride, or the like. In this case, however, steps for applying the adhesive to the packaging material 11 and drying it are additionally required, increasing the complexity and costs in the production process and making the work troublesome.